Organelle
thumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles]] In cell biology, an organelle is a discrete structure of a cell having specialized functions. There are many types of organelles, particularly in the eukaryotic cells of higher organisms. An organelle is to the cell what an organ is to the body (hence the name organelle, the suffix ''-elle'' being a diminutive). Organelles were historically identified through the use of microscopy, and were also identified through the use of cell fractionation. A few large organelles probably originated from endosymbiont bacteria: * mitochondria (in almost all eukaryotes) * plastids (in plants and algae) ** chloroplasts, mature forms of etioplasts ** chromoplasts ** leucoplasts ** amyloplasts *** statoliths ** elaioplasts ** proteinoplasts ** rhodoplasts Other organelles have had endosymbiotic origins suggested for them (notably flagella; see Evolution of flagella), but these theories are not widely accepted. Eukaryotic organelles Eukaryotes are the most structurally complex known cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost cell membrane. The larger organelles, such as the nucleus and vacuoles, are easily visible with moderate magnification (although sometimes a clear view requires the application of chemicals that selectively stain parts of the cells); they were among the first biological discoveries made after the invention of the microscope. Not all eukaryotic cells have all of the organelles listed below, and occasionally, exceptional species of cells are missing organelles which might otherwise be considered universal to eukaryotic cells (such as mitochondria). There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below (e.g. some which are listed as double-membraned are sometimes found with single or triple membranes). Organelles which have double-membranes and their own DNA are believed by many biologists of having originally come from incompletely consumed or invading prokaryotic cells, which were adopted as a part of the invaded cell through endosymbiosis. Originally, the word organelle referred to large lipid bags within cells; later, as other cell parts were discovered, the meaning was extended to also include smaller parts of cells. Other related structures: * cytosol * endomembrane system * nucleosome * microtubule * cell membrane Prokaryotic organelles Prokaryotes are not as structurally complex as eukaryotes, and do not have any compartments enclosed by lipid membranes. In the past they were often viewed as having little internal organization, but slowly details are emerging about prokaryotic internal structures. One contributing discovery was that at least some prokaryotes have microcompartments, which are compartments enclosed by proteins. See also *Cell *Endosymbiotic theory References * Alberts, Bruce et al. (2002). The Molecular Biology of the Cell, 4th ed., Garland Science, 2002, ISBN 0-8153-3218-1. * Kerfeld, Cheryl A et al., Protein Structures Forming the Shell of Primitive Bacterial Organelles, Science 309:936-938 (5 August 2005). Category:Cell biology ar:عضية bn:অঙ্গাণু bg:Клетъчни органели cs:Buněčné organely a struktury da:Organel de:Organell es:Orgánulo eo:Organeto fa:اندامک fr:Organite ko:세포소기관 is:Frumulíffæri it:Organulo he:אברון lb:Organell lt:Organoidas mk:Органела ms:Organel nl:Organel ja:細胞小器官 no:Organelle pl:Organellum pt:Organelo ro:Organite celulare ru:Органоиды sk:Organela su:Organél sv:Organell tr:Organel uk:Органела zh:細胞器